enso/docs/debugger/mixed-debugging.md

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# Debugging Enso and Java Code at Once
Enso libraries are written in a mixture of Enso code and Java libraries.
Debugging both sides (the Java as well as Enso code) is possible with a decent
IDE.
Get [NetBeans](http://netbeans.apache.org) version 13 or newer or
[VS Code with Apache Language Server extension](https://cwiki.apache.org/confluence/display/NETBEANS/Apache+NetBeans+Extension+for+Visual+Studio+Code)
and _start listening on port 5005_ with _Debug/Attach Debugger_ or by specifying
following debug configuration in VSCode:
```json
{
"name": "Listen to 5005",
"type": "java+",
"request": "attach",
"listen": "true",
"hostName": "localhost",
"port": "5005"
}
```
Then it is just about executing following Sbt command which builds CLI version
of the engine, launches it in debug mode and passes all other arguments to the
started process:
```bash
sbt:enso> runEngineDistribution --debug --run ./test/Tests/src/Data/Numbers_Spec.enso
```
Alternatively you can pass in special JVM arguments when launching the
`bin/enso` launcher:
```bash
enso$ JAVA_OPTS=-agentlib:jdwp=transport=dt_socket,server=n,address=5005 ./built-distribution/enso-engine-*/enso-*/bin/enso --run ./test/Tests/src/Data/Numbers_Spec.enso
```
As soon as the debuggee connects and the Enso language starts - choose the
_Toggle Pause in GraalVM Script_ button in the toolbar:
![NetBeans Debugger](https://user-images.githubusercontent.com/26887752/209614191-b0513635-819b-4c64-a6f9-9823b90a1513.png)
and your execution shall stop on the next `.enso` line of code. This mode allows
to debug both - the Enso code as well as Java code. The stack traces shows a
mixture of Java and Enso stack frames by default. Right-clicking on the thread
allows one to switch to plain Java view (with a way more stack frames) and back.
Analyzing low level details as well as Enso developer point of view shall be
simple with such tool.